U.S. patent number 6,377,851 [Application Number 09/484,868] was granted by the patent office on 2002-04-23 for implantable cardiac stimulation device and method for optimizing sensing performance during rate adaptive bradycardia pacing.
This patent grant is currently assigned to Pacesetter, Inc.. Invention is credited to Jim C. Chen, Eric S. Fain, Anthony Mo, Mae-Mae Shieh.
United States Patent |
6,377,851 |
Shieh , et al. |
April 23, 2002 |
Implantable cardiac stimulation device and method for optimizing
sensing performance during rate adaptive bradycardia pacing
Abstract
An implantable cardiac stimulation device including a
ventricular defibrillator and a rate adaptive cardiac pacer
automatically adjusts post-pacing sensing parameters dependent upon
pacing rate. The device includes a pulse generator that applies
stimulation pulses to a heart at a calculated variable stimulation
rate as a function of physiologic demand. A sensing circuit senses
ventricular activity of the heart responsive to a plurality of
sensing parameters including post-pace sensing parameters and a
processor adjusts the post-pace sensing parameters responsive to
the selected pacing rate.
Inventors: |
Shieh; Mae-Mae (Fontainebleau,
FR), Chen; Jim C. (New York, NY), Mo; Anthony
(Fremont, CA), Fain; Eric S. (Menlo Park, CA) |
Assignee: |
Pacesetter, Inc. (N/A)
|
Family
ID: |
26868036 |
Appl.
No.: |
09/484,868 |
Filed: |
January 18, 2000 |
Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N
1/39622 (20170801); A61N 1/362 (20130101); A61N
1/37 (20130101); A61N 1/3956 (20130101) |
Current International
Class: |
A61N
1/362 (20060101); A61N 1/39 (20060101); A61N
001/362 () |
Field of
Search: |
;607/4,5,9 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Medtronic GEM DR Dual Chamber ICD System, Model 7271, The Logical
Choice; Medtronic 1998, 4 pages..
|
Primary Examiner: Getzow; Scott M.
Attorney, Agent or Firm: Mitchell; Steven M.
Parent Case Text
This application claims the benefit of U.S. Provisional Application
No. 60/172,389, filed Dec. 17, 1999.
Claims
What is claimed is:
1. An implantable cardiac stimulation device including a
ventricular defibrillator and a rate adaptive cardiac pacer
comprising:
pulse generating means for applying stimulation pulses to a
patient's heart at a variable stimulation rate that is a function
of a sensed physiologic demand;
sensing means for sensing ventricular activity of the heart
responsive to a plurality of sensing parameters including
post-stimulation sensing parameters including an initial sensing
threshold; and
adjusting means for adjusting the initial sensing threshold
responsive to a calculated stimulation rate.
2. The device of claim 1 further including processing means for
determining the post-stimulation sensing parameters responsive to
the calculated stimulation rate.
3. The device of claim 1 wherein the device includes a look-up
table including the post-stimulation sensing parameters versus
stimulation rate for adjusting the post-stimulation sensing
parameters responsive to the calculated stimulation rate.
4. The device of claim 3 further including processing means for
providing the look-up table with the post-stimulation sensing
parameters versus stimulation rate.
5. The device of claim 4 wherein the processing means provides the
look-up table with the post-stimulation sensing parameters versus
stimulation rate based upon fixed criteria.
6. The device of claim 4 wherein the processing means provides the
look-up table with the post-stimulation sensing parameters versus
stimulation rate based upon selectable criteria.
7. An implantable cardiac stimulation device including a
ventricular defibrillator and a cardiac pacer comprising:
a pulse generator that applies stimulation pulses to a heart at a
variable stimulation rate;
a sensing circuit that senses ventricular activity of the heart
responsive to a plurality of sensing parameters including
post-stimulation sensing parameters;
a processor that adjusts the post-stimulation sensing parameters
responsive to a calculated stimulation rate; and
wherein the sensing circuit has an initial sensing threshold,
wherein the initial sensing threshold is one of the
post-stimulation sensing parameters and wherein the processor
adjusts the initial sensing threshold responsive to the calculated
stimulation rate.
8. The device of claim 7 wherein the processor is programmed to
determine the post-stimulation sensing parameters responsive to the
calculated stimulation rate.
9. The device of claim 7 further including a memory having a
look-up table including the post-stimulation sensing parameters
versus stimulation rate.
10. The device of claim 9 wherein the processor is programmed to
provide the look-up table with the post-stimulation sensing
parameters versus stimulation rate.
11. The device of claim 10 wherein the processor is programmed to
provide the look-up table with the post-stimulation sensing
parameters versus stimulation rate based upon fixed criteria.
12. The device of claim 11 wherein the processor is programmed to
provide the look-up table with the post-stimulation sensing
parameters versus stimulation rate based upon selectable
criteria.
13. In an implantable stimulation device including a ventricular
defibrillator and a rate adaptive cardiac pacer, a method of
applying stimulation pulses to a heart and sensing ventricular
activity after applying a stimulation pulse including the steps
of:
applying stimulation pulses to a heart at a stimulation rate that
varies as a function of physiologic demand;
sensing ventricular activity of the heart responsive to a plurality
of sensing parameters including post-stimulation sensing
parameters; and
adjusting one of the post-stimulation sensing parameters comprising
an initial sensing threshold responsive to a calculated stimulation
rate.
14. The method of claim 13 wherein one of the post-stimulation
sensing parameters is an initial sensing threshold and wherein the
adjusting step includes adjusting the initial sensing threshold
responsive to the calculated stimulation rate.
15. The method of claim 13 wherein the sensing step includes
decreasing a sensing threshold a delay time after applying a
stimulation pulse, wherein the delay time is one of the
post-stimulation sensing parameters, and wherein the adjusting step
includes adjusting the delay time responsive to the calculated
stimulation rate.
16. The method of claim 13 further including the step of
determining the post-stimulation sensing parameters prior to
calculating the selected stimulation rate.
17. The method of claim 13 including the further step of providing
a look-up table including the post-stimulation sensing parameters
versus stimulation rate and wherein the adjusting step includes
obtaining the post-stimulation sensing parameters from the look-up
table responsive to the calculated stimulation rate.
18. The method of claim 17 including the further step of providing
the look-up table with the post-stimulation sensing parameters
versus stimulation rate.
19. The method of claim 18 wherein the step of providing the
look-up table with the post-stimulation sensing parameters versus
stimulation rate is performed based upon fixed criteria.
20. The method of claim 18 wherein the step of providing the
look-up table with the post-stimulation sensing parameters versus
stimulation rate is performed based upon selectable criteria.
21. An implantable cardiac stimulation device including a
ventricular defibrillator and a rate adaptive cardiac pacer
comprising:
pulse generating means for applying stimulation pulses to a
patient's heart at a variable stimulation rate that is a function
of a sensed physiologic demand;
sensing means for sensing ventricular activity of the heart
responsive to a plurality of sensing parameters including
post-stimulation sensing parameters including a decreasing sensing
threshold which begins decreasing a delay time after application of
a stimulation pulse; and
adjusting means for adjusting the delay time responsive to the
calculated stimulation rate.
22. An implantable cardiac stimulation device including a
ventricular defibrillator and a cardiac pacer comprising:
a pulse generator that applies stimulation pulses to a heart at a
variable stimulation rate;
a sensing circuit that senses ventricular activity of the heart
responsive to a plurality of sensing parameters including
post-stimulation sensing parameters;
a processor that adjusts the post-stimulation sensing parameters
responsive to a calculated stimulation rate; and
wherein the sensing circuit has a decreasing sensing threshold
which begins decreasing a delay time after application of a
stimulation pulse, wherein the delay time is one of the
post-stimulation sensing parameters, and wherein the process
adjusts the delay time responsive to the calculated stimulation
rate.
23. In an implantable stimulation device including a ventricular
defibrillator and a rate adaptive cardiac pacer, a method of
applying stimulation pulses to a heart and sensing ventricular
activity after applying a stimulation pulse including the steps
of:
applying stimulation pulses to a heart at a stimulation rate that
varies as a function of physiologic demand;
sensing ventricular activity of the heart by setting a sensing
threshold and decreasing the sensing threshold a delay time after
applying a stimulation pulse; and
adjusting the delay time responsive to a calculated stimulation
rate.
Description
FIELD OF THE INVENTION
The present invention generally relates to an implantable cardiac
stimulation device including both a ventricular defibrillator and a
rate adaptable cardiac pacemaker. The present invention more
particularly relates to such a device and method providing enhanced
post-pacing sensing performance.
BACKGROUND OF THE INVENTION
Combined implantable ventricular defibrillator and pacemaker
stimulation devices are well known in the art. Such devices permit
a heart to be paced for treating bradycardia, for example, while
also detecting for ventricular fibrillation and ventricular
tachycardia and applying defibrillating electrical energy,
cardioversion shocks or antitachycardia pacing pulses to the heart
when fibrillation or tachycardia is detected.
One problem that must be addressed in such devices is the need to
provide relatively low threshold, i.e., high sensitivity,
ventricular sensing for detecting fibrillation while pacing the
heart. The sensing threshold must be low enough (sensitive enough)
for detecting the low amplitude electrical activity of the heart
during fibrillation while avoiding over-sensing which could result
in a T wave being detected by the pacemaker and thus mistaken for
an R wave. The foregoing is most notably a problem after a pacing
stimulation pulse is applied to the heart by such devices.
In the prior art, post-pacing sensing has been performed by first
establishing a ventricular refractory period (VREF) when the pacing
stimulation pulse is applied and continuing the VREF for a
pre-determined time through the evoked response. Following the
VREF, the sensing threshold is set at an initial level, held at the
initial level for a delay time, and then decreased thereafter from
the initial threshold level to a minimum threshold level where it
is held until the next paced or sensed event. The initial
threshold, delay time and threshold decay rate are selected so that
the threshold is above the amplitude of the T wave when the T wave
occurs.
These post-pace sensing parameters can be varied to achieve the
desired sensing threshold characteristics. For increased
sensitivity to low level signals, as occur during fibrillation, it
is desirable for the threshold to decrease to the minimum threshold
as quickly as possible before the next pace pulse. However, to
prevent over-sensing of larger T waves, particularly in patients
with longer QT intervals, it is desirable for the sensing threshold
to be higher or less sensitive. Therefore, the most optimal set of
post-pace sensing parameters is the one which achieves the desired
threshold level without over-sensing T waves. This problem is
further complicated when rate adaptive pacing is implemented. Rate
adaptive pacing is used with patients whose heart rates do not
naturally increase in response to exercise (chronotropic
incompetence). The rate adaptive pacer senses a physiologic
parameter indicative of exercise and provides a corresponding
increase in the pacing rate. However, this reduces the time between
stimulation pulses and thus the time during which the sensing
threshold can decrease to ensure the detection of low-level
fibrillation signals. The time between pacing pulses is also
shortened in a P-wave tracking mode for those patients whose hearts
are not chronotropically incompetent. P-waves are sensed in the
atria and the ventricle(s) is paced at the rate which tracks the
P-waves and thus at a rate that may increase as a result of
exercise or excitement. As used herein, the term "rate adaptive" is
intended to include pacing at a rate that varies in response to
some change in physiological condition whether that be P-wave
tracking, response to a sensor measuring exercise or otherwise.
Further, since the QT interval generally shortens with faster
pacing rates (and conversely lengthens with slower rates), a single
post-pace sensing parameter set cannot yield the most optimal
thresholding for all pacing rates. Another complicating factor is
the variability of QT intervals and T wave amplitudes between
patients and differing conditions. Prior art sensing systems have
not addressed this problem of faster pacing rates in a rate
adaptive pacer reducing the amount of time available for the
threshold to decrease, compounding the problem of achieving the
desired threshold (or sensitivity) by the next pace pulse.
The present invention addresses the problem of achieving the
optimal thresholding during variable pacing rates. The present
invention achieves the optimal thresholding without requiring
complicated programming of the device by the patient's
physician.
SUMMARY OF THE INVENTION
The invention provides an implantable stimulation device including
a ventricular defibrillator and a rate adaptive cardiac pacer which
optimizes sensing performance following application of pacing
pulses to a heart. The device includes a pulse generator that
applies pacing stimulation pulses to a patient's heart at a
stimulation rate that is a function of physiologic demand. The
device further includes a sensing circuit that senses ventricular
activity of the heart for supporting pacing of the heart and
fibrillation detection. The sensing circuit senses ventricular
activity in accordance with a plurality of sensing parameters
including post-stimulation sensing parameters. The device further
includes a processor that adjusts the post-stimulation sensing
parameters responsive to the stimulation rate.
The invention still further provides a method of applying
stimulation pulses to a heart and sensing ventricular activity
after applying a stimulation pulse to the heart. The method
includes the steps of applying stimulation pulses to a heart at a
stimulation rate that is a function of physiologic demand, sensing
ventricular activity of the heart responsive to a plurality of
sensing parameters including post-stimulation sensing parameters,
and adjusting the post-stimulation sensing parameters responsive to
the calculated stimulation rate.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and advantages of the present
invention will be more apparent from the following more particular
description thereof, presented in conjunction with the following
drawings and wherein;
FIG. 1 shows a simplified functional block diagram illustrating an
implantable cardiac stimulation system in which the present
invention may be used;
FIG. 2 shows a functional block diagram of an implantable combined
cardioverter/defibrillator and pacemaker device embodying the
present invention;
FIG. 3 shows a functional block diagram of the 10 chip of the
device of FIG. 2;
FIG. 4 shows a functional block diagram of the controller of the
device of FIG. 2;
FIG. 5 is a waveform of a paced cardiac cycle illustrating
particular aspects of the present invention; and
FIG. 6 is a look-up table which may be used to adjust the post-pace
sensing parameters in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is of the best mode presently
contemplated for carrying out the invention. This description is
not to be taken in a limiting sense, but is made merely for the
purpose of describing the general principles of the invention. The
scope of the invention should be determined with reference to the
claims.
Referring now to FIG. 1, the implantable cardiac stimulation system
10 there illustrated includes a dual chamber implantable
cardioverter-defibrillator (ICD) 12 and an external programmer 14.
The ICD is preferably of the type that includes both a
cardioverter/defibrillator and a rate adaptive pacemaker. The
pacemaker may be, in accordance with the present invention, either
a single chamber ventricular pacemaker or a dual chamber
pacemaker.
The implantable device or ICD is coupled to a patient's heart 16 by
a plurality of electrode carrying leads 18, 20, and 22. The lead 18
is preferably an atrial pacing and sensing lead having a bipolar
electrode pair of the type well known in the art positioned in the
right atrium of the patient's heart 16 in a known manner. The lead
20 preferably is a ventricular pacing and sensing lead having a
bipolar electrode pair, also of the type well known in the art,
positioned in the right ventricle of the patient's heart 16. As is
well known, an additional pacing and sensing lead (not shown) may
be employed having a bipolar electrode pair positioned in the
coronary sinus to provide left side sensing and pacing of the
heart. Lastly, the lead 22 is preferably a defibrillation lead
having a first or proximal shock coil electrode positioned in the
superior vena cava (SVC) of the patient's heart and a second or
distal shock coil electrode positioned in the right ventricle of
the patient's heart as is well known in the art. In a preferred
embodiment all of the conductors and electrodes are combined in a
single lead or in some cases two leads with the elements of leads
20 and 22 combined.
The leads 18 and 20 support dual chamber pacing of the heart 16 and
sensing of electrical activity of the right atrium and right
ventricle respectively of the heart 16. Such sensing allows the
dual chamber pacemaker to stimulate the right atrium and right
ventricle only when necessary on demand, i.e., in the absence of a
sensed intrinsic depolarization. The sensing provided by lead 20
also provides for detection of tachyarrhythmias, such as
ventricular fibrillation and ventricular tachycardia of the heart
16. Lastly, the lead 22 permits defibrillation or cardioversion
shocks to be applied to the heart when fibrillation or tachycardia
is detected. The defibrillation shocks are preferably applied from
the combination of the conductive housing 24 and the proximal shock
coil in the SVC to the distal shock coil in the right ventricle.
Alternatively, the lead may be provided with only one shock coil
for positioning in the right ventricle. With this arrangement, the
defibrillation shocks may be applied between the conductive housing
24 of the ICD 12 and the right ventricular shock coil, as is known
in the art.
The ICD 12 detects activity of the heart, provides stimulation
pacing pulses and/or high voltage shocks to the heart to treat
tachyarrhythmias and bradycardia of the heart in accordance with a
plurality of programmable parameters. The programmable parameters
are provided by the programmer 14 under selective operation by the
patient's physician. The programmable parameters are conveyed by
the programmer 14 to the ICD 12 over a telemetry link 28 in a known
manner.
The ICD 12 relies on accurate sensing to expediently detect low
level fibrillation signals while at the same time avoiding
over-sensing of T waves during bradycardia pacing. As will be seen
hereinafter, the ICD 12 includes a sensing circuit that senses
activity of the heart in accordance with a plurality of sensing
parameters. In accordance with the present invention, the sensing
parameters include post-pacing sensing parameters which are
automatically adjusted by the ICD 12 depending on the calculated
pacing stimulation rate of the rate adaptive pacer. The
post-stimulation sensing parameters are preferably obtained by the
ICD 12 from a look-up table residing in a static RAM memory 54
within the ICD 12 as this is more energy efficient than calculating
these parameters in the ICD microprocessor. Alternatively the
post-stimulation sensing parameters may be determined for each of a
plurality of anticipated stimulation rate ranges by a processor 15
within programmer 14 or by a processor within the ICD 12. If the
programmer determines the parameters, it loads the look-up table
with the determined parameters by transmitting the parameters to
the ICD 12 over the telemetry link 28.
The post-stimulation sensing parameters define the value of the
post-stimulation sensing threshold used to sense ventricular
activity using lead 20. As may be seen in FIG. 5, the sensing
threshold is set to an initial or start value 30 at the end of a
ventricular refractory period (VREF) which is imposed following the
application of a pacing stimulation pulse 32. As mentioned above,
VREF may be fixed or may vary as a function of the pacing rate. The
R-wave feature 34 is the evoked response of the heart resulting
from the stimulation pulse 32. The sensing threshold may then be
held constant for a decay delay 36 depending on the calculated
stimulation rate. It is noted that in the preferred embodiment the
ICD microprocessor calculates during VREF when the next pacing
pulse is expected to be delivered based on sensor input, P-wave
tracking rate and/or other parameters. It then determines the
post-stimulation sensing parameters, also within the VREF period.
In an alternative embodiment, the microprocessor may adaptively
determine when the next stimulation pulse is to be delivered on an
on-going basis including additional parameters such as, for
example, the morphology of the evoked response. The duration of the
decay delay,if there is one, is further dependent on the calculated
pacing rate of the rate adaptive pacer. Following the decay delay
36, the sensing threshold is decreased at a prescribed decay rate
until it reaches a minimum threshold 31 prior to the time for the
next stimulation pulse 38. In the preferred embodiment, the
prescribed decay rate remains constant regardless of the selected
stimulation rate. Other decay functions such as a step function or
a parabolic decay can be used to decrease the sensing
threshold.
In accordance with the present invention, the post-stimulation
sensing parameters which are adjusted to the calculated rate
responsive pacing rate include the initial threshold 30 and the
decay delay 36. Since the prescribed decay rate is the same for
each pacing rate, by adjusting the decay delay 36 and the initial
threshold 30, responsive to the pacing rate, over-sensing causing
detection of a T wave may be avoided. In FIG. 5, sensing of the T
wave 40 is avoided since the threshold is above the amplitude of
the T wave when the T wave occurs.
FIG. 6 shows an illustrative look-up Table 42. The look-up Table 42
preferably resides within a memory within the ICD 12. As will be
noted, for each pacing rate range, the look-up Table 42 defines a
value of a decay delay and start threshold. As previously
mentioned, the look-up Table 42 may not call for a decay delay at
some pacing rates. The reason for this is that at higher pacing
rates, the QT interval may be short enough so that the T-wave falls
within VREF. This renders a decay delay to be unnecessary. A manner
of determining the look-up table parameters will be described
subsequently. In an alternative embodiment (not shown) the look-up
table may include other variable post-stimulation sensing
parameters such as a variable VREF or a variable decay rate.
Further, the start threshold in the look-up table is used in the
case of a paced event. In the case of a sensed event, the pulse
generator determines the maximum amplitude signal detected during
VREF. Upon expiration of VREF, the sensing threshold is set to a
programmed percentage of the detected maximum amplitude.
Referring now to FIG. 2, the ICD 12 includes within the
electrically conductive housing 24 a controller chip 50, a
read-only memory (ROM) chip 52, a static random access memory
(SRAM) chip 54, and an IO chip 56. The ICD 12 further includes a
high voltage controller (HVC) chip 58, and high voltage output
stage 60, a battery 62, and high voltage capacitors 64.
The above-mentioned chips form an ICD hybrid. The hybrid is
connected through a system of address and data buses 66 forming a
highly specialized, computerized, embedded system.
The controller 50, to be described in greater detail subsequently,
provides the main control of the ICD 12 and determines its
functionality. The controller 50 is coupled to the ROM 52 which
contains the software of the ICD 12. This software includes
operating instructions which the controller 50 executes to control
the operation of the ICD 12.
The SRAM 54 may contain the aforementioned look-up table defining
the adjustable post-pace sensing parameters and the programmable
parameters of the ICD 12. It also preferably includes buffers for a
stored intracardiac electrogram (SIEGM) subsystem.
The IO chip 56 regulates the sensing function of the ICD 12. To
this end, it receives over the address and data buses 66 the
sensing parameters including the post-pace sensing parameters. The
10 chip provides trigger pulses over a bus 68 which causes delivery
of pacing stimulus pulses to the heart. It further receives
electrogram signals from the atrial and ventricular leads 18 and 20
respectively over another bus 70. When the 10 chip detects a
cardiac event within the right atrium or right ventricle, it
generates an interrupt on a bus 72. The interrupts are used for
timing and diagnostic purposes.
The HVC chip 58 controls the high voltage output stage 60. It
receives data from the controller 50 over buses 66 defining the
magnitude of electrical energy to be delivered to the heart for
cardioversion and defibrillation therapy. It further receives
interrupts from the 10 chip 56 over bus 72 to control the timing of
therapy delivery.
The high voltage output stage 60 is coupled to the battery 62 and
the HV capacitors 64. It includes DC-to-DC converter circuitry
which convert the DC battery voltage to a relatively high voltage
for charging the high voltage capacitors 64. Under control of the
HVC chip 58, the high voltage output stage 60 discharges the
capacitors into the heart for terminating detected
tachyarrhythmias. The cardioversion/defibrillation energy is
applied to lead 22 over a conductor 74.
As previously mentioned, the IO chip 56 regulates the sensing
function of the ICD 12. The IO chip 56 is shown in greater detail
in FIG. 3. The IO chip 56 generally includes a morphology stage 80,
a sensing stage 82, a measured data stage 84 and a pacing stage
86.
The morphology stage 80 and sensing stage 82 receive a common input
over buses 70 from the atrial and ventricular leads 18 and 20
respectively (FIG. 1). The morphology stage 80 analyzes the
electrogram signals to help distinguish supraventricular
tachycardias from ventricular tachycardias.
The sensing stage 82 includes sense amplifiers (not shown) of the
type well known in the art for amplifying and filtering the raw
electrogram signals. One of the sense amplifiers is a ventricular
sense amplifier whose sensitivity or threshold is controlled by the
controller 50 (FIG. 2) which uses the sensing parameters, including
the post-pacing sense parameters, contained in the look-up table
stored in the SRAM 54. When the sensing stage 82 detects an R wave,
it provides an interrupt on conductor 88. The interrupt notifies
the controller chip 50 (FIG. 2) of intrinsic activity. The
frequency of the interrupts are utilized by the controller to
determine if antibradycardia, antitachycardia, or defibrillation
therapy is necessary.
The measured data stage 84 performs measurements of various ICD
parameters. These parameters include the voltage of the battery 62,
the voltage on capacitors 64, the lead impedance of the
defibrillation lead 22 (FIG. 1) and the lead impedance of the
atrial and ventricular sense/pace leads 18 and 20 respectively
(FIG. 1). The measured data stage provides an interrupt over line
90 to notify the controller 50 when a measurement has been
completed.
The pacing stage 86 generates stimulation pulses for bradycardia
therapy at an output 92 and antitachycardia pacing therapy at an
output 94. The sensing stage 82 is coupled to the pacing stage 86
to cause the pacing stage 86 to generate stimulation pacing pulses
only in the absence of intrinsic activity. The pacing stage 86
generates stimulation pacing pulses having a specified amplitude
and width as controlled by the controller 50.
FIG. 4 is a more detailed block diagram of the controller 50. The
controller 50 includes a processor 100, interrupt logic 102, timers
104, a watchdog subsystem 106, and control and test registers 108.
The controller 50 further includes a telemetry circuit or system
110 and an IEGM storage system 112.
The processor 100 is the main component of the controller 50. It
and the other subsystems identified above facilitate the
implementation of the ICD functions.
Telemetry circuit 110 is configured to provide full duplex
communication with the external programmer 14 (FIG. 1). It provides
bidirectional data paths for downloading programmable parameters,
including post-pacing sensing parameters in accordance with the
present invention, and instructions to the ICD 12 (FIG. 1). In
accordance with one aspect of the present invention, the programmer
14 may determine the post-pacing sensing parameters and transmit
them to the ICD 12 via the telemetry circuit 110, for storage by
the processor 100 in the look-up table of the SRAM 54 (FIG. 2).
Alternatively, the processor 100 may determine the post-pacing
sensing parameters for storage in the look-up table of SRAM 54
after receiving predicted factors for QT interval prediction from
the programmer via the telemetry circuit 110. The telemetry circuit
110 further permits the programmer to interrogate the ICD to
determine status information and to retrieve the SIEGM stored in
the IEGM storage system 112. The pacing rate in the look-up table
42 is also programmable.
The IEGM storage system 112 consists of SRAM chips where the
digitized IEGM is stored as is disclosed in U.S. Pat. No. 5,732,708
which is incorporated herein by reference. The stored data may be
transferred to the programmer through the telemetry circuit 110 for
further analysis.
The timers 104 provide accurate time measurements and event
processing. The timers 104 interrupt the processor 100 when they
expire and require processor service.
The watchdog subsystem 106 provides a safety mechanism against
runaway software processes. For example, if the processor 100 is
held in an infinite loop, the watchdog system will cause a system
wide reset to occur.
The control and test registers 108 allow various software processes
to change the hardware configuration by changing the contents of
programmable registers. Interrupts may be enabled, disabled or
acknowledge by using registers within the control and test
registers 108.
The pacing rate dependent post-pace sensing parameters may be
calculated based upon the alert period, the predicted QT interval
at that rate, the maximum T wave amplitude anticipated and the
threshold level desired before the next pacing pulse is delivered.
More specifically, the following equations may be implemented by
either the processor 15 of the programmer 14 (FIG. 1) or the
processor 100 (FIG. 4) of the ICD 12 to determine the rate
dependent post-pace sensing parameters.
Where QT_Scaling_Factor=9609.7 and exp=0.7056
Where Max Sens=0.6
Decay delay=(Predicted QT-VREF)-(Start threshold-T
wave)*(15.6/0.05) Eqn. (3)
Where
For a Pacing Rate Bin of 130 to 150, T wave=1.0
For a Pacing Rate Bin of 110 to 120, T wave=1.15
For a Pacing Rate Bin of 100, T wave=1.3
For a Pacing Rate Bin of 90, T wave=1.4
For a Pacing Rate Bin of 30 to 80, T wave=1.5
With respect to equation 2, for a Pacing Rate Bin of 100 to 150, if
calculated start threshold is greater than 1.5, the start threshold
is set equal to 1.5. For a Pacing Rate Bin of 30 to 90, if the
calculated start threshold is greater than 1.6, start threshold is
set equal to 1.6. If the calculated start threshold is less than
0.9, the start threshold is set equal to 0.9. Further, the
following conditions have precedence over the calculated decay
delay: i. If predicted QT is less than VREF, then the predicted QT
interval is within refractory and decay delay 0. ii. If start
threshold is less than T Wave, then the decay delay=0. iii. If the
pacing rate is 150, then the decay delay=0.
The alert period, predicted QT intervals, maximum T wave amplitudes
anticipated, and the threshold level desired before the next pace
pulse may be prestored for use in determining the rate dependent
post-pace sensing parameters. Alternatively, and in accordance with
the present invention, the determination process may be customized
by a user if oversensing of T waves is experienced or if there is a
desire to increase sensing sensitivity. In accordance with this
aspect of the present invention, the user may select via the
programmer the T wave amplitude, QT interval, and pacing rate with
which oversensing is observed to determine the rate dependent
post-pace sensing parameters using the relationships given above.
Either the programmer or the ICD will then recalculate the
appropriate parameters to prevent oversensing and to achieve the
desired sensitivity before the next pacing pulse is to be
delivered. This method of inputting clinical parameters to modify
the sensing threshold behavior is more intuitive to the user as
compared to directly changing the post-pace sensing parameters.
While the invention herein disclosed has been described by means of
specific embodiments and applications thereof, numerous
modifications and variations could be made thereto by those skilled
in the art without departing from the scope of the invention set
forth in the claims.
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